Black enamel frit obscurations are commonly applied to automotive glazing. The practice of applying black enamel frit obscurations became common place the 1980s when the industry switched from the rubber H channel and the butyl adhesive strip windshield mounting systems to polyurethane adhesive. This change was made in response to the poor safety performance of the prior technology. The channel/butyl mounted windshields were often dislodged on impact allowing the occupants to be ejected from the vehicle. To improve occupant retention in crashes, the industry to switched to polyurethane adhesive mounting. Today, on new vehicles, butyl is no longer used and rubber H channel mount is only seen on commercial and off-road vehicles.
One advantage of the butyl strip system was that the butyl strip was narrow enough to be covered with a molding or trim strip to obscure the butyl and the mounting flange. The rubber channel also did not require an obscuration as the rubber covered both the edge of glass and the flange.
This had to change to enable the use of polyurethane. To get the required strength, it was found that the bead of polyurethane had to be substantially wider than the butyl strip had been. With a minimum width of 19 mm, it was no longer practical to obscure the adhesive from view from the outside with a trim strip or molding due to the width that would have been required. In addition, polyurethane needs to be protected from ultra-violet light, UV, to prevent degradation. As a result, the black enamel frit band was added to the glass, to obscure the view of the polyurethane and to protect the polyurethane by blocking the UV. This obscuration band of black enamel frit is commonly called the “black band”.
While there can be no doubt that the polyurethane system has saved countless lives, it comes at price.
Black enamel frit is comprised of pigments, a carrier, binders and finely ground glass. Other materials are also sometimes added to enhance certain properties: the firing temperate, anti-stick, chemical resistance, etc. The frit is applied to the glass using a silk screen or ink jet printer prior to heating and bending. During the bending process, the ground glass soften and fuses with the glass surface. The frit is said to be “fired” when this takes place. This is very similar to the process used to apply enamel finishes on bathroom fixtures and appliances.
Metals and many other types of materials have an ultimate yield strength specified at which the material will always fail. However, with glass we can only specify a probability of breakage for a given value of stress. Looking at glass at the molecular level, we would expect the strength to be very high. In fact, what we find in practice is that glass has a very high compressive strength, as expected, but very low tensile strength.
For a given set of identical glass specimens, if each is flexed to the point of breakage, the breaking point might appear to be a random variable. In fact, the yield point follows a Weibull distribution and the probability of breakage can be calculated from the test data as a function of stress, duration, area, surface defects and the modulus of glass.
To the naked eye, float glass appears to be near perfect. Any defects that may be present are so small as to not be visible to the naked eye. But, in fact, at the microscopic level, the surface can be seen to be rough and dotted with flaws. When the glass is placed in tension, these surface defects tend to open up and expand, eventually leading to breakage and failure. Therefore, glass always fails in tension. Even when not placed in tension, the surface defects will react with moisture and slowly “grow” over time. This phenomenon is known as slow crack growth.
For these reasons, it is desirable to minimize any damage to the surface during handling and processing and to also minimize any residual stress in the glass.
A fired black frit increases surface defects. This can be seen if the black pigment of a fired black band is chemically dissolved. The surface of glass will have a frosted appearance, similar to sand blasted or chemically etched glass. The frosted appearance is due to the myriad surface defects present from the fused glass. This makes the surface weaker increasing the probability of breakage.
Another problem arises from the thermal gradient that results from the black frit during the bending process. As one would expect, the black frit absorbs heat better than clear glass. The painted area tends to run hotter than the adjacent clear areas. The abrupt thermal gradient on the surface, going from the black to the clear area of the glass, results in optical distortion along the inner edge of the black band. This is known as the “burn” line in the industry.
One method used to address the burn line problem is the dot fadeout. Starting at the inner solid edge of the black paint, rows of increasingly smaller dots are painted on the glass. This is the same principle as used in grayscale printing. This reduces the rate of change in the surface temperature, spreading the gradient over a wider area and reducing the rate of change. The dot fadeout also hides the distortion. However, on some parts, even a wide dot fadeout is not sufficient to eliminate all distortion. A wide dot fadeout also may not be allowed depending upon the size of the opening and the regulatory requirements for driver vision.
In addition, the black frit interferes with the annealing process. Windshield glass must be annealed to meet safety requirements. Glass is annealed by heating the glass to a temperature in the glass transition range to soften it, letting it soak for some period of time to allow any stress to be relieved, and then slowly cooling through the glass transition range until the glass freezes. The hotter black tends to freeze after the cooler clear portion resulting in residual stress in the laminate.
Another problem caused by the black frit is surface mis-match. A laminate is comprised of at least two layers of glass. The frit is typically applied to only one of the glass layers. This can result is a slight difference in the shape of the surfaces. When the two surfaces are forced together during lamination, the mis-match results in residual stress in the laminate.
The area of the windshield with a black frit obscuration has increased in recent years.
To improve aerodynamics and lower wind noise, the wiper rest position on many vehicles is located out of sight below the hood line. When this is done, the bottom edge of the windshield is extended to form a wiper rest 31 so as to provide a smooth transition resting surface for the wipers when not in use as shown in FIGS. 1, 2 and 3. This area is typically obscured with a black enamel frit 32 extending downward and connecting to the black band. The width of this area can be in excess of 15 cm. This area is also normally heated, to prevent snow and ice from building up, using a screen print conductive silver on the fourth surface of the glass 24. The power density required to keep this area free of ice and snow is often double that of a rear window defroster due to the snow and ice that is packed in from the wiper action. The wide wiper rest frit obscuration, with drawbacks intrinsic to frit, coupled with the thermal stress of melting snow and ice, leads to a high breakage rate.
As the electronic content of modern vehicles has increased, the area of the windshield, near the top center, has become increasingly crowded on many vehicles. Once the province of just the rear-view mirror, we now find a wide array of equipment mounted in this location.
One of the first devices to compete for this area was the infra-red rain sensor used to provide for a full automatic mode of windshield wiper operation as well as other vehicle functions such as closing the sun roof and pulsing the brakes to keep the rotors dry. The rain sensor must have a lens optically coupled to the fourth surface of the windshield 24. An IR LED shines light onto the first surface. The amount of light reflected back correlates to the amount of water present on the first surface 21. The field of view of the lens must be in the area cleared by the wipers in order to function properly. Therefore, the lens is generally mounted on or near the vertical centerline near the top of the wiped area. Power and signal wiring is required for the sensor to work. In addition to the lens 10, the sensor comprises a housing 12 and a cover 14 all of which need to be hidden from view from the outside of the vehicle to avoid an unsightly cluttered appearance.
The rear view mirror itself has undergone a major transformation. Once a purely mechanical device, electro-chromic automatic dimming rear-view mirrors have become a popular option and are standard equipment on many models. The additional components required to provide this functionality make the mirror heavier and therefore require a larger footprint mounting bracket than a standard non-dimming mirror. Power also needs to be provided to the mirror. A cover is often provided to protect the cable and to hide it from the inside of the vehicle.
The rear-view mirror has further evolved as telematics driver aid systems and hands free blue-tooth interfaces have been introduced. The rear view mirror is a convention place to locate control push-buttons and microphones which require additional wires which also must be protected and covered.
The use of cameras, requiring a wide field of view and a high level of optical clarity, are also growing at a rapid rate. These typically must be mounted on the windshield in the wiper area. Initial applications were for night-vision. Today, camera based systems are used to provide a wide array of safety functions including adaptive cruise control, obstacle detection, lane departure warning and more. Many of these applications require the use of two cameras. A clear undistorted field of view, with minimal double imaging, is especially critical for camera based systems to perform as intended. It is essential for these systems to be able to quickly differentiate between objects, capture text, identify signage, and operate with minimal lighting.
While covers 14 and various styling methods can be used to obscure the components and cables from the inside, it is also required to maintain clean lines and a good aesthetic when the vehicle is viewed from the exterior.
Standard practice has been to extend the black enamel frit band 32 to create an obscuration 8 on the fourth surface 24 with openings in the obscuration 30 to provide for the required field of view as shown in FIGS. 1, 2 & 3.
When the black enamel frit band 32 is extended downward from the top center black band to create an obscuration 8 on the fourth surface 24, distortion and stress can become a major problem. This is because the black frit is extending further from the edge into the area where more heat must be applied to bend the glass. The large surface area of the obscuration increases the probability of breakage due to the surface defects and stress introduced by the frit. This is also a critical viewing area for windshield mounted cameras. If the distortion cannot be minimized, the opening for the camera must be made larger than would otherwise be required to provide an undistorted field of view.
A panoramic windshield is a windshield in which the top edge of glass has been extended to include at least a portion of the roof giving the driver a vertical field of vision of at least 45 degrees as defined by the applicable regulatory standards.
In the case of a panoramic windshield FIG. 3, the problem is even more pronounced as the obscuration 8 is located several cm from the top edge at or near the center of the windshield, the weakest point of the windshield. This is also where the highest temperatures are required to bend the glass.
A laminated windshield is comprised of two glass layers permanently bonded together by a thin plastic layer 4.
Automotive laminate interlayers and inks were commercialized in 2003. Some of the key patents include: U.S. Pat. No. 8,002,938 B2, U.S. Pat. No. 7,232,213 B2 and U.S. Pat. No. 7,278,730. To date, the primary application has been for architectural laminates, providing a means to produce vivid colors and patterns at 600 DPI that were never before possible. However, little has been done in the automotive market. DE201110004500 teaches the advantages of a printed plastic layer for use in an automotive laminate for reducing weight and providing artistic freedom to the designer.
One of the reasons why they have not been used in automotive windshields is that screen print enamel frits provides a high degree of protection from UV which is needed to protect the polyurethane used to bond the glazing to the vehicle opening. Inks printed on the interlayer do not have the opacity and UV blocking ability that the black enamel frits do.